1. Field of the Invention
The present invention relates to a movable object position detecting method and apparatus, and more particularly to such a method and apparatus for use in an absolute type of encoder which detects an absolute position of a movable object.
2. Description of the Related Art
A typical one of the conventional absolute type of encoders is constructed by a light source, a code plate which is attached to a movable object whose position is to be detected and which has in the direction of movement of the movable object a plurality of tracks in which binary code patterns are formed, and a plurality of light detector elements arranged opposite to the plural tracks, whereby output signals of the light detector elements are detected to determine a position of the code plate.
There is known another absolute type of encoder formed by a combination of the above-mentioned position detection system using the binary code patterns and an incremental position detection system. In this type of encoder, a rough absolute position of the code plate is detected with a predetermined detection width by means of the binary code patterns and a precise position of the code plate within the predetermined detection width is detected in the incremental mode.
Further, JP-A-No. 61-189415 discloses a scale position detecting apparatus which comprises an absolute position detection part using binary code patterns for detecting an absolute position of a scale and a high resolution position detection part for detecting a position of the scale within the minimum detection width of the absolute position detection part by using a phase splitting technique.
Furthermore, Japanese Patent Application No. 61-300909 assigned to the assignee of this application has proposed a movable object position detecting apparatus which comprises a first-order position detection part for detecting the absolute position of a code plate attached to the movable object by means of patterns regularly arranged in the code plate, and a second-order position detection part for lower digits lower than those of the first-order position detection part for detecting a position of the code plate within the minimum detection width of the first-order position detection part, by means of inclined slits, which are formed in the code plate at a periodical pitch of arrangement equal to the minimum detection width of the first-order position detection part and making an angle of .theta.(0.degree.&lt;.theta..ltoreq.90.degree.) with the direction of movement of the code plate, and slits, which are formed in a mask to be opposite to the inclined slits formed in the code plate, the relation between the first and second order position detection parts being repeated successively.
The above-mentioned encoders or position detecting apparatuses involve the following problems.
Namely, in the case of the first-mentioned encoder using the binary code patterns, the improvement of the precision of position detection requires the formation of an extremely great number of tracks and the provision of the corresponding number of light detector elements. Therefore, this encoder has a problem that complicated processing of signals is required and the cost of the encoder becomes very high.
Though the second-mentioned encoder based on the combination of the position detection system using binary code patterns and the incremental detection system has a merit that relatively less numbers of binary code patterns and light detector elements suffice, thereby making it possible to perform position detection with high precision by an inexpensive structure, an operation of this encoder requires detecting a rough position of the code plate with a predetermined detection width by using binary code patterns, thereafter detecting a reference position set within the predetermined detection width by searching a reference signal pattern, and then incrementally detecting an amount of displacement from the detected reference position to a position to be detected. Thus, this encoder has a problem that the operation becomes complicated, thereby making a response speed slow.
The above-mentioned position detecting apparatus formed by a mere combination of the absolute position detection part using binary code patterns and the high resolution position detection part based on the phase splitting technique has a problem that, as a detection range is made wider, the pitch of slits formed in the high resolution position detection part becomes greater correspondingly, thereby lowering the precision of detection by the phase splitting technique. Further, there is a problem that, unless the boundary of a detection range in the absolute position detection part and the boundary of a detection range in the high resolution detection part are respectively made to coincide with each other with precision higher than a precison with which the position detection should be made, an error may be produced in detection data at those boundaries.
The fourth-mentioned position detecting apparatus using the inclined slits also has a problem that a detection error may be produced due to the discontinuity at the boundary between the higher-order and lower-order position detection parts.